GEOS-6218: enabled CSV outputformat for complex features with flat attributes

https://jira.codehaus.org/browse/GEOS-6218

This is to enable CSV outputformat for flat complex features. It doesn't currently work with propertyName selection (requires another patch in GetFeature). Documentation was also updated.

Also, this will only work for portrayal complex features (SF0), e.g. geosciml-portrayal. It's difficult to build the CSV header with unbounded multi-valued properties in true complex features.

The unit test is in app-schema-test to avoid having to add gt-complex/gt-app-schema dependencies into wfs module.